When a conventional electronic stability control system detects a loss of steering control, the brakes are automatically applied to assist in steering the vehicle. Braking force is applied to the wheels individually, such as the outer front wheel to counter over steer or the inner rear wheel to counter under steer. Electronic stability control systems may also reduce engine power until control is regained.
Active braking systems, or differential braking systems, apply different braking forces to each of the four wheels of a vehicle to produce a different braking force between the left and right wheels.
Referring to FIG. 1a, one example of an active braking system is illustrated. A drive train 10 includes a traction engine or traction motor 12 that drives a drive shaft 14 with a torque of 2τd that is provided to an open differential 16. A right wheel 18 is driven by a right axle 20 that provides torque τd to the right wheel to impart angular velocity ωR to the right wheel 18. A left wheel 24 is driven by a left axle 26 that provides torque τl to the left wheel 26 to impart angular velocity ωL to the left wheel 26. Braking force τb is illustrated to be applied to the left wheel 24 for electronic stability control.
Referring to FIG. 1b, an over steering situation is illustrated for a vehicle 30. The vehicle is traveling on an over steer path x but it is intended to be following the desired path y. The electronic stability control system is shown to be applying a braking force τb to the right front wheel to create a yaw moment as indicated by the arrow m. The difference in applied braking forces applied may generate yaw moments under a wide range of conditions of vehicle operation. The total torque distribution provided to the outside wheels is limited to 50% torque transfer from the motor.
The problems and shortcomings of the above systems are addressed by the disclosed system as summarized below.